The intrathecal administration of the anticancer agent mercaptopurine (i.e. directly into the cerebrospinal fluid, CSF, of the central nervous system) may provide an effective method for the treatment of acute lymphocytic leukemia (ALL). Such treatment requires careful control of drug levels in the CSF. With high speed digital computers it may be possible to use a sophisticated model of mercaptopurine kinetics in conjunction with a mathematical algorithm for dosage selection to rapidly and effectively control the CSF concentration of mercaptopurine. Towards this goal the metabolism of 6-MP have been investigated in monkeys following intrathecal and intravenous administration of mercaptopurine. The major finding of the research has been the development of a physiological-pharmacokinetic model of mercaptopurine kinetics in the CSF. The salient feature of the model is that nearly all of the 6-MP that enters the CSF from the plasma does so via newly formed CSF. As a result of this observation, new experiments are being conducted on the use of the internal cartoid artery as a means of delivering 6-MP to the brain. The methodology is currently being tested on monkeys.